1. Field of the Invention
The present invention relates to a multi-axis magnetic lens and variants thereof used for focusing a plurality of charged particle beams individually and in parallel. More particularly, it relates to a multi-axis magnetic lens acting as an objective lens or a condenser lens or a transfer lens in a multi-beam apparatus which uses a plurality of electron beams to in parallel expose patterns onto or inspect defects on a wafer or mask in semiconductor manufacturing industry. Compared with a conventional single-beam counterpart of the multi-beam apparatus, it can obtain a much higher throughput without degrading spatial resolution.
2. Description of the Prior Art
In semiconductor manufacturing industry, an electron beam has been used to expose patterns onto or inspect defects on a wafer or a mask when critical feature dimensions of patterns or defects are beyond the competent ability of a photon beam. The reason is that an electron beam can offer superior spatial resolution compared to a photon beam due to its short wavelength. However, such a superior spatial resolution will be fundamentally deteriorated by electron interaction or called as Coulomb Effect as the electron beam current is increased to obtain a throughput competent for mass production.
To mitigate the limitation on throughput, instead of using one electron beam with a large current, it is proposed many years ago to use a plurality of electron beams each with a small current to expose patterns onto a wafer in parallel, such as U.S. Pat. No. 3,715,580. For structuring a multi-beam apparatus using a plurality of electron beams, one critical problem is how to separately focus multiple electron beams individually and in parallel. Configuring multiple conventional single-beam columns (MSCs) into one multi-beam apparatus was a first solution naturally thought out. Because the spatial interval between every two adjacent beams must be large enough to physically accommodate two single-beam columns in parallel, the number of electron beams available for a wafer or a mask is not sufficient for mass production. As an alternate to using the MSCs, configuring a multi-axis lens to individually focus multiple electron beams in parallel is a promising way to be able to use more electron beams. Compared with the structure of the MSCs, a multi-axis lens will reduce the beam interval by 50%, thereby almost doubling the apparatus throughput.
In U.S. Pat. No. 3,715,580, Maekawa et al. propose a multi-axis magnetic lens for throughput improvement of an IC pattern exposure system. The multi-axis magnetic lens is schematically shown in FIG. 1A, which comprises a common excitation coil 44, one yoke 43 and two magnetic conductor plates 41 and 42 with a plurality of through round holes in pairs. When an electric current is exerted into the coil 44, a magnetic round-lens field will be generated between each pair of coaxial holes respectively in the plates 41 and 42. By this means, multiple magnetic sub-lenses are therefore formed, such as 10, 20 and 30. Each magnetic sub-lens has an optical axis coincident with the coincident central axes of two coaxial holes (such as 31 in FIG. 1B), and can focus an electron beam (such as 1 in sub-lens 10, 2 in sub-lens 20 and 3 in sub-lens 30) entering the sub-lens along the optical axis thereof.
In the foregoing multi-axis magnetic lens, the magnetic flux leakage between each pair of coaxial holes depends on the positions thereof on the plates 41 and 42, geometrical shapes and magnetic permeability of the plates 41 and 42, and the distribution of all the holes on the plates 41 and 42. Hence, the magnetic fields of all the sub-lenses are fundamentally not a pure round-lens field and different from each other in distribution pattern and strength as shown in FIG. 1B. Consequently, there are two inherent issues which hinder all the electron beams to obtain superior resolutions similar to that of a conventional single beam focused by a single-axis lens.
The first issue is a non-axisymmetry of the magnetic field in each sub-lens. The magnetic field distribution of each sub-lens degenerates from axial symmetry to a rotation symmetry and/or n-fold symmetry. In terms of Fourier analysis, the magnetic field comprises not only an axisymmetric component or called as round-lens field, but also a lot of non-axisymmetric transverse field components or called as high order harmonics, such as dipole field and quadrupole field. Only the round-lens field is necessary for focusing an electron beam, and the other components are undesired due to their impairment on beam focusing. The dipole field deflects the charged particle beam, thereby making the beam land on the image plane with position error, additional tilt angle and deflection aberrations, while the quadrupole field adds astigmatism to the beam focusing. To compensate the influence of each high order harmonic, at least one additional element generating the same type field is required for each electron beam.
The second issue is the focusing power differences among all the sub-lenses if all the through round holes are same in geometry. The round-lens fields of all the sub-lenses are not equal to each other due to the differences in magnetic flux leakage. The sub-lens closer to the geometrical center of the plates 41 and 42 has a weaker round-lens field. For instance, compared with the sub-lenses 10 and 30, the sub-lens 20 has a weaker round-lens field. Due to the round-lens field differences, the beams 1, 2 and 3 respectively passing through the sub-lens 10, 20 and 30 are focused onto different image planes, not a same image plane.
Many scientists propose methods to fundamentally mitigate or even eliminate the two issues per se. Lo et al. in U.S. Pat. No. 6,750,455 uses a plurality of dummy holes to improve the local structure symmetry of each sub-lens. However this method makes the multi-axis magnetic lens system bulky. Chen et al. in U.S. Pat. No. 8,003,953 forms a permeability-discontinuity (simply expressed as PD hereafter) unit inside each hole of every sub-lens to eliminate non-axisymmetric transverse field components inside every sub-lens and the focusing power difference among all the sub-lenses. For the sake of clarity, the foregoing unit is named as the first-type PD unit hereafter. Abstractly speaking, the first-type PD unit comprises non-magnetic and magnetic annular layers in alternate arrangement, i.e. a magnetic annular layer is immediately enclosed by a non-magnetic annular layer and/or immediately encloses a non-magnetic annular layer. Inside every hole where a first-type PD unit is formed, the outermost layer adjoins the inner sidewall of the hole, and the innermost layer is a magnetic annular layer and becomes a pole-piece of the sub-lens formed by the hole. Concretely speaking, one or more magnetic rings with high permeability are inserted into each hole of every sub-lens and separated by a non-magnetic radial gap from each other so as to form multiple coaxial layers. From the inner sidewall of the hole to the innermost layer of the unit (i.e. the innermost magnetic ring), permeability at least alternately decreases and increases spatially one time. The innermost magnetic ring is the pole-piece of the sub-lens formed by the hole. FIG. 2A exemplifies a simple embodiment of the first-type PD unit, which takes the sub-lens 30 in FIG 1A as an example and renames it as 30-1 for the sake of clarity.
In FIG. 2A, two magnetic rings 32 and 33 both having high permeability are respectively inserted into the two coaxial holes in magnetic conductor plates 41 and 42 with two radial gaps G1 and G2. The two gaps G1 and G2 are either a vacuum space or filled with a non-magnetic material. On the one hand, inside the hole of the plate 41, one first-type PD unit is formed by the gap G1 and the magnetic ring 32, and consequently permeability spatially decreases from permeability u41 of the magnetic conductor plate 41 to 1 and then increases to permeability u32 of the magnetic ring 32. On the other hand, inside the hole of the plate 42, one first-type PD unit is formed by the gape G2 and the magnetic ring 33, and consequently permeability spatially decreases from permeability u42 of the magnetic conductor plate 42 to 1 and then increases to permeability u33 of the magnetic ring 33. The magnetic rings 32 and 33 therefore constitute two pole-pieces of the sub-lens 30-1. A magnetic field along the optical axis 31 is generated through the non-magnetic gap between these two pole-pieces 32 and 33. The upper pole-piece 32 is extended into the inner hole 33h of the lower pole-piece 33 to eliminate the non-axisymmetric transverse field components in the gap. The thicknesses of gaps G1 and G2 on the one hand have to be small enough to keep a sufficient magnetic coupling for making the round-lens field strong enough, and on the other hand large enough to minimize non-axisymmetric transverse field components to a negligible level inside the inner holes 32h and 33h of the upper and lower pole-pieces 32 and 33 respectively. The non-axisymmetric transverse field components generated outside the sub-lens are reduced by two magnetic tubes 36 and 37. In such a way, the non-axisymmetric transverse field components in the areas inside and outside each sub-lens are reduced to a level much lower than that in FIG. 1A. The round-lens field differences or called as focusing power differences among all the sub-lenses are eliminated by specifically choosing thickness differences of the gap G1 and/or the gap G2 among all the sub-lenses.
Based on the fundamental of the multi-axis magnetic lens in U.S. Pat. No. 8,003,953, Chen et al. further propose a multi-axis magnetic immersion objective in the cross-reference, which comprises a plurality of immersion objective sub-lenses so that a plurality of charged particle beams can be individually and in parallel focused onto a specimen surface with small aberrations. One embodiment is shown in FIG. 2B, which also takes the sub-lens 30 in FIG. 1A as an example and renames it as 30-2 for the sake of clarity. Two magnetic shielding plates 50 and 51 with a plurality of through round openings sandwich the magnetic conductor plates 41 and 42 with two axial gaps G11 and G12. The magnetic rings 32 and 33 are respectively inserted into a pair of coaxial holes on plates 41 and 42 with two radial gaps G1 and G2 to form two first-type PD units therein and become two pole-pieces of the sub-lens 30-2. The gaps G1 and G2 are either a vacuum space or filled with a non-magnetic material. The lower ends of two pole-pieces 32 and 33 are configured to form a radial gap G3 opposite to the specimen 60 and extended inside the corresponding coaxial hole in the lower magnetic shielding plate 50.
As well known, a new multi-axis magnetic lens is always desired if it is easier in manufacturing and at least not worse in performance than the prior art. Accordingly, increasing simplicity and flexibility of the first-type PD unit in configuration and manufacturing is needed in reducing ease and cost of manufacturing.